1. Field of the Invention
The present invention concerns an electron spin resonance system using a permanent magnet magnetic circuit as a magnetic field generating device and, more particularly, it relates to an electron spin resonance system (hereinafter referred to as "ESR system") having a structure, which is micro-miniaturized by combining a permanent magnet magnetic circuit using a movable yoke for continuously varying a distance between yokes opposite to each other thereby continuously adjusting a magnetic field in a desired gap, and a microwave generator/receiver.
The ESR system is generally used for chemical analysis in the field of research and development.
Since the ESR system can detect unpaired electrons due to natural radiation damages of test objects and exactly measure the ages of archaelogical remains, minerals and fossil remains, it has also been utilized recently in the field of archaelogy and earth science.
The ESR system for such application uses comprises a magnetic field generating device and a microwave circuit device connected to it and it is necessary, for the magnetic field generating device forming an essential part of the ESR system, that it can form a magnetic field of such an intensity as determined depending on the physical property value of a test object, frequency of microwave, etc., at high accuracy and uniformity in a gap in which a test object is placed.
2. Description of the Prior Art
In the magnetic field generating device used for the ESR system, electromagnets have usually been employed. It is easy to generate a magnetic field in a desired gap and continuously change the intensity thereof by continuously varying a current applied to the electromagnets but this involves a drawback that the entire device weight of the system reaches as much as 1 to 2 ton and the system becomes expensive as well.
Further, the microwave circuit device also has a complicate structure to make the size larger and the cost more expensive.
Further, there has been proposed a structure using only permanent magnets as a magnetic field generating source so that the magnetic field intensity is changed by changing the opposing distance (length of the gap) between a pair of permanent magnets.
However, this structure also has a disadvantage that when the opposing distance is changed, the degree of uniformity of the magnetic field is impaired due to various causes such as deviation of the axis of one of the opposing permanent magnets from that of the other.
Usually, the degree of uniformity of the magnetic field required in the ESR system is as high as less than 0.01% and, accordingly, a device using a magnetic circuit comprising permanent magnets has no yet been put to practical use.
In view of the above, the present applicant has already proposed a constitution for a magnetic field generating device capable of continuously changing the intensity of the magnetic field in a desired gap without changing the opposing distance between permanent magnets, that is, a constitution in which a permanent magnet is disposed to each of opposing surfaces of a pair of stationary yokes disposed opposite to each other with a gap, a movable yoke is disposed to at least one of a pair of the stationary yokes for continuously varying the distance between the opposing surfaces of the stationary yokes, and the magnetic resistance of a magnetic path formed by the permanent magnets the stationary yokes and the movable yoke is continuously varied by the operation to the movable yoke thereby enabling to adjust the magnetic field intensity in the gap (refer to Japanese Utility Model Laid-Open Hei 1-104574).
The applicant has also proposed a magnetic circuit for generating a magnetic field of a constitution capable of varying the intensity of the magnetic field in a desired gap at a high accuracy, in which a movable yoke is disposed to each of a pair of stationary yokes described above such that the movable yokes are brought closer to or aparted from each other in symmetry with respect to the central traverse plane of the gap (refer to Japanese Patent Application Hei 1-154875).
With the same purpose, the applicant also has proposed a magnetic circuit for generating magnetic field, in which the movable yoke disposed to each of the stationary yokes comprises a plurality of members which are disposed, for example, coaxially and made movable individually, to attain finer adjustment for the intensity of the magnetic field (refer to Japanese Patent Application Hei 1-218233).
These proposals have enabled the employment of a magnetic circuit using permanent magnets as a magnetic field generating source for the ESR system.
However, for improving the practical usefulness of the ESR system, it is necessary, in addition to the improvement of the magnetic circuit described above, that the magnetic circuit is arranged effectively and that the entire ESR system including peripheral equipments, etc. for reading the value of a magnetic field causing magnetic resonance at high accuracy has to be reduced in the size and the weight.